The present invention relates to a machine for grinding the circumference of ophthalmic lenses.
Grinding machines already exist for use in the ophthalmic field for edging, bevelling or grooving ophthalmic lenses. Edging is an operation which enables the outer contour or circumference of an ophthalmic lens, which is generally circular in shape, to be matched to the contour of the frame into which the lens is to be fitted. The operation thus consists in removing certain portions of the lens periphery.
Bevelling of an ophthalmic lens, which is carried out after edging, enables the transverse cross-section of the peripheral edge of the lens to be matched to the shape of the groove normally provided in spectacle frames in order that the lens may be fitted thereinto. Bevelling thus consists of providing a rib or bevel of a generally triangular shape on the outer edge of the lens.
Grooving of an ophthalmic lens is an operation which is also carried out after edging of the lens. Its purpose is to adapt the cross-section of the outer edge of the lens in order that the lens can be fitted into certain types of frame. Indeed, certain frames are not provided with a groove over the whole inner perimeter of the frame; thus, for, example, the frame may only correspond to the upper portion of the lens whereas the lower portion of the lens is only held in place by means of a transparent filament. In this case, it is necessary to provide a groove or channel, most frequently of a triangular or semi-circular cross-section, on the outer edge of the lens.
Most frequently, these three operations are carried out on a single grinding machine fitted with a set of grinding wheels. Such machines are known and are available commercially. One such example is described in French patent No. 2,543,039 in the name of the present applicant, equivalent to U.S. Pat. No. 4,596,091.
These grinding machines generally consist of a frame carrying, firstly, one or several diamond tipped grinding wheels often linked together to constitute a grinding wheel train, and rotatably mounted about an axis, and, secondly, a swinging head frame for carrying the lens. This head frame is fitted with gripping means adapted to receive, retain and rotate the ophthalmic lens to be processed. In such a grinding machine, the head frame and the grinding wheel or wheels are adapted to move relative to each other firstly in the direction of the grinding wheel axis (axial movement) and, secondly, substantially perpendicularly to said axis (so-called orthoradial movement). Relative axial movement enables changeover from one grinding wheel to another to be achieved as well as enabling the contour of the lens circumference to be followed during bevelling or grooving operations. Substantially orthoradial relative movement enables lenses which are non-circular in shape to be machined. This present invention relates to the substantially orthoradial movement of the swinging head frame with respect to the grinding wheels in a grinding machine.
Most frequently, the grinding wheel or wheels are rotatably mounted on a first axis and the head frame is slidably and pivotally mounted about a second axis parallel to the first axis. The head frame is sometimes also mounted so as to move perpendicularly to the axis of the grinding wheel or wheels.
In known machines, pressing means urge the head frame towards the grinding wheel axis. These pressing means rely on gravity alone or the combined effect of gravity and means such as springs or a counterweight system. Thus, in the Applicant's patent cited above, the pressing means urging the head frame towards the grinding wheel axis rely on the effect of gravity and a spring having adjustable tension.
In conventional machines, a template having the desired final shape of the lens is simultaneously mounted with the lens on the axis of rotation thereof. A vernier is mounted so as to be immovable with respect to the grinding wheel axis, and includes a feeler at its end enabling the point when the vernier comes in contact with the template to be determined. In more recent machines, the template is replaced by a disk and the vernier moves substantially along its horizontal axis as a function of the shape of the lens to be obtained. A feeler provided at the end of the vernier similarly allows the point when the vernier comes into contact with the disk to be determined. In both cases, when the feeler of the vernier comes into contact with the template or the disk at a given angular position of the lens, this means that machining is completed in this angular position.
Thus, during the whole machining operation, in other words during edging, bevelling or grooving, the lens is in abutment with one of the grinding wheels, under the action of the pressing means. Contact between the feeler of the vernier and the template or disk enables the precise moment at which machining is terminated to be determined at each angular position.
This system according to the prior art suffers from certain disadvantages. Firstly, the head frame is simply supported by the lens resting on the grinding wheel during use, under the action of the pressing means. Because of this, there is a danger that the head frame can be subject to uncontrollable vibrations which the pressing means, simply consisting of the force of gravity or the force provided by elastic urging means or counterweights are not able to eliminate.
Moreover, the pressing means apply a constant force to the lens in the case of a counterweight, and a force that is substantially constant when an elastic spring means is used. This may, in the case of high-power plastic material lenses, lead to a force that is too high being exercised on the grinding wheel causing the motor to come to a standstill. In the case of very thin lenses, the opposite applies and this constant load may be too high causing the lens to shatter.
Moreover, known systems necessitate complicated and somewhat irrational machining cycles to be employed. Thus, one machining mode consists in selecting one direction of rotation of the lens, and in changing the direction of rotation each time the feeler on the vernier comes in contact with the template or disk.
Finally, because of its very construction, this type of system is intrinsically limited as regards its accuracy due to the need for the feeler provided at the end of the vernier to travel a certain minimum distance before an output signal is issued from the detection means associated therewith.